The onset of exogenous feeding in marine fish larvae

The onset of exogenous feeding in fish larvae can be considered as the period from which the ingestion is possible up to the moment when larval growth is detected. The main characteristic of this phase is that the source of nutrient and energy necessary to continue the larval development changes from the yolk reserves to the ingested food. To achieve this transition with success it is necessary that all structures and organs related with food uptake, digestion and assimilation are ready in due time and that the appropriate food is available. This review will focus on the most relevant processes during this short period of early life history of marine fish: gut anatomy, digestive capacities, feeding behaviour and metabolism.At the opening of the mouth and anus the yolk is completely or almost exhausted. The gut is a simple tube histologically differentiated in foregut, midgut and hindgut. The pancreas, liver and gall bladder are functional. From the first moment of feeding the larvae are able to ingest, digest and assimilate food particles. The digestion starts in an alkaline environment with the contribution of pancreatic enzymes as well as cytosolic enzymes. The main limitations at the beginning of exogenous feeding are mouth gape, restricting the particle size and larval length, restricting swimming capacity and hunting success. After the opening of the mouth, the organogenesis continues. A quick growth and differentiation of the digestive tract is necessary to reinforce digestion and nutrients absorption. A few days later the larval length and jaw size have increased enough to allow a more effective predation.